1. Field of the Invention
This invention concerns a porous film and a method of manufacturing the same. More specifically, it relates to a porous film having excellent moisture permeability, flexibility and exudation resistance. Further, it relates to a manufacturing method capable of forming and stretching the porous film having the characteristics described above at a high speed.
2. Statement of Related Art
Various methods of manufacturing porous films by stretching at least in one direction films formed by adding inorganic fillers and various kinds of oils and waxes to polyolefin resins and causing voids due to interface peeling between the inorganic filler and the polyolefin have been proposed so far. Since the porous films obtained by the methods have many minute voids at the inside, they do not allow liquids to permeate therethrough while having high moisture permeability. Utilizing the above-mentioned property, they have been used in a wide range including sanitary materials such as disposable paper diapers, building materials such as water proof sheets, reflection films and electric cell separators.
However, such films containing a great amount of inorganic fillers involve a drawback upon preparation and stretching in that film formation and stretching at high speed are difficult. That is, incorporation of the inorganic filler worsens draw down, to cause breakage and fracture of films, pinholes upon high speed film formation and stretching to hinder the improvement of the productivity. This trend is remarkable when the thickness of the film is reduced and it has been difficult to produce uniform films by film formation and stretching at high speed. Further, when the porous films are used as the material for the disposable diapers, it has been demanded for the performance of not permeating urine or the like but allowing only the moisture to permeate therethrough. Depending on the water absorptive polymers used in the disposable diapers, a slight amount of surface active agents may leach in the absorbed urine. In such a case, urine tends to be oozed from the porous film.
Among the problems described above, Japanese Patent Publication No. 35734/1993 proposes a method of adding castor oil as third ingredient to resin composition comprising polyolefin resin and inorganic filler for improving the uniformness of thickness and stretching/molding property at high speed. However, although the uniform thickness and the stretching/moldability at high speed can be improved to some extent according to this method, it still leaves a problem in view of the exudation from the film. Further, Japanese Patent Publication No. 99581/1994 proposes a porous film containing a mixture of hydrocarbon polymer and polyester as third ingredient. However, although the proposed porous film has favorable uniform thickness and flexibility, it involves a problems in the exudation resistance of the film. Furthermore, Japanese Patent Publication No. 15090/1987 proposes a porous film containing liquid or wax hydrocarbon polymer as third ingredient. However, although this porous film is improved for the exudation resistance, it still involves a problem in the uniformness of thickness and the flexibility.